A structure having multiple fine salient portions of a scale smaller than the wavelength of light (hereinafter also referred to as “fine structure”) has heretofore been used as various structural materials such as a super-water-repellent material, a super-hydrophilic material, a battery material, an antifriction material, and an antireflection material.
Processes for producing such fine structure having multiple fine salient portions of a scale smaller than the wavelength of light can be roughly classified into the following three processes:
(1) a process involving forming a product to serve as a mask on a product to be processed in advance and then etching the product to be processed (involving a mask-placing step and an etching step);
(2) a process involving etching a product to be processed (involving only the etching step); and
(3) a process involving subjecting the product to be processed to transfer molding with a fine structure produced by the process (1), (2), or the like as a mold.
Production by a semiconductor process including a main lithography technology has been attempted as the conventional production example (1) including the mask-placing step and the etching step (Patent Literature 3). The process involves: applying a resist to a substrate to be used in fine structure formation; exposing and developing the resist through a photomask to provide a mask pattern; and transferring the mask pattern onto the substrate for fine structure formation through etching. The lithography technology is advantageous as a process by which a precisely designed fine pattern can be formed (Patent Literature 1).
A technology involving spontaneously forming the mask pattern without employing the lithography technology has also been proposed. For example, there has been disclosed a technology by which a fine structure of a scale smaller than the wavelength of light can be easily produced with fine particles arranged on a substrate, an island-shaped metal thin film, or the like as a mask (Patent Literatures 2 and 3).
A phenomenon in which a needle shape is formed in an Si plasma etching (reactive ion etching) process has been known as the conventional production example (2) formed “only of the etching step,” and the material is called “black silicon” (Non Patent Literature 1).
For example, a process involving transferring a fine protrusion shape onto a photocurable resin or the like with a carbon material having fine protrusions, which is formed by using a plasma etching apparatus as a mold has been proposed as the conventional production example (3) involving transferring the fine structure as a mold (Patent Literature 4).
In particular, when the shapes of the salient portions in the fine structure are cone shapes, i.e., the salient portions is of such a shape that the area of a cross section perpendicular to a direction from its bottom portion to its tip reduces along the direction, the fine structure can be used as, for example, an antireflection film, a water-repellent film, a super-hydrophilic film, a substrate for mass spectrometry, a battery material, or an antifriction material.
A technology involving suppressing reflection with a light interference effect obtained by laminating one or more optical films each having a refractive index different from that of a substrate so that the thickness of the laminate may be several tens to several hundreds of nanometers is general as a technology concerning an antireflection film. A vacuum deposition process such as vapor deposition or sputtering, or a wet deposition process such as dip coating or spin coating is employed for forming the optical films. Such general antireflection film has a light beam incident angle of 0° and is designed to have an excellent antireflection effect in a wavelength region where the film is used, which is a relatively narrow wavelength region. However, an antireflection film to be used in, for example, a lens having a large aperture or a lens having a surface with a small radius of curvature is desired to have an excellent antireflection function for a wide wavelength region and to have a good incident angle characteristic for light flux. The arrangement of a fine structure whose pitch is shorter than the wavelength of incident light (referred to as, for example, “sub-wavelength structure (SWS)” or “moth-eye structure”) has been known as an antireflection measure available in a wide wavelength region and having a good incident angle characteristic (Patent Literatures 5 and 6). In particular, when the fine structure has a sharpened structure, a volume occupied by a substance in a space gradually increases from the upper portion of a salient structure to its lower portion, and hence an abrupt change in refractive index at an interface is suppressed. As a result, the reflection is largely reduced.
The fact that a film having, on its surface, a fine structure formed of multiple salient portions shows high water repellency has been known as a lotus effect (Patent Literature 7 and Non Patent Literature 2). Such super water repellency that the contact angle of a water droplet at the surface of the film exceeds 150° can be realized by properly designing conditions such as the shapes, arrangement, and hydrophobic modification groups of the salient portions. The film showing such super water repellency has been expected to find applications in, for example, surface coating for various optical members and building members.
The film having, on its surface, the fine structure formed of the multiple salient portions can be used as a substrate for mass spectrometry as well. The foregoing is a technology involving holding a material to be analyzed on a semiconductor thin film having the fine structure, applying laser or an ion beam to ionize the object to be analyzed without destroying its structure, and measuring its mass with high sensitivity to identify a component of the material, and the technology has been known as surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) similar to matrix-assisted mass spectrometry (Patent Literature 8). The fine structure on the surface of the substrate mainly has a role of preventing the destruction of the specimen sample in association with the ionization and hence needs to be of a size of about 10 nm. Porous silicon produced by the anodization of silicon is most general as the substrate to be used in surface-assisted laser desorption/ionization mass spectrometry. In addition, for example, a titanium oxide thin film having a fine structure on its surface, the thin film being produced by causing a polyethylene glycol to coexist or by a sol-gel process, has been proposed as a material capable of solving the problem of the deterioration of a characteristic resulting from a reduction in electric conductivity of the porous silicon due to oxidation in air (Non Patent Literature 3). Random-shaped fine pores of about 10 nm are present in the titanium oxide thin film to be used in the foregoing and these fine pores each mainly have a role of preventing the destruction of the specimen sample in association with the ionization.